1. Field of the Invention
This invention relates to a process of manufacturing vitreous beads in which particles of a vitrifiable material are vitrified and spherulized.
2. Background of the Art
Vitreous beads fall broadly into two main categories, namely solid beads and hollow beads. The term "hollow beads" as used herein denotes beads having a void therein having a relative density less than 1.0, such that those beads float on water whereas the term "solid beads" having no void therein denotes beads having a relative density equal to or greater than 1.0 such that these beads do not float on water. Beads in both categories find wide use as fillers for plastics materials for various purposes. Hollow beads are also used as fillers for certain explosives, especially those based on an aqueous emulsion, in order to increase their brisance, and deuterium filled hollow beads also find use as laser fusion targets. Solid beads are also incorporated in paint for the manufacture of reflective signs, for example road signs, and for shot- or sand-blasting.
It is well known to manufacture solid glass beads by spherulizing crushed glass cullet, for example originating from waste offcuts from a flat glass manufacturing plant. However problems of economic production will arise if there is no waste glass of suitable composition available for use as a feedstock. The production of glass for use as feedstock in such processes requires the use of a glass melting furnace operating at high temperatures, depending on the glass composition, at which the refractory material of the furnace is subject to considerable erosion; also the fuel costs for heating the furnace are very high.
In contrast to that simple process which is useful for forming solid glass beads, hollow beads are usually formed from a feedstock comprising pellets of a glass-former composition based on sodium silicate which may have been reacted with some other ingredient such as boric acid. These particles are vitrified and spherulized in a spherulizing furnace. The glass-former composition contains a substance which gives rise to the evolution of gas in such furnace with consequent cellulating effect.
The production of hollow vitreous beads for use as laser fusion targets, starting from a fragmented silica or silica-based gel, is referred to in "Processing of Gel Glasses" by Jerzy Zarzycki in "Glass Science and Technology" (Ed. D. P. Uhlmann and N. J. Kreidl, The Academic Press Inc. 1984) Volume 2, pages 213 to 245. The gel contains occluded water which evaporates when the particles are heated to bring about their vitrification and spherulization, so generating vapour pressure which has an expanding and cellulating effect on the glass beads.
It is an object of the present invention to provide a process by which vitreous beads can be economically and conveniently produced from a glass-former composition and which is at the same time applicable for producing both solid and hollow beads.
According to the present invention, there is provided a process of manufacturing vitreous beads in which particles of a glass-former composition containing chemically bound water, are fed through a heating zone in which the particles are vitrified and spherulized and the resulting vitreous beads are cooled, and wherein the size range of said particles and their content of substance which becomes gaseous during the passage of the particles through said heating zone are such that at least 20%, and preferably at least 30%, by weight of the formed beads have a relative density greater than 1.0.
A particular advantage in forming solid beads by a process according to the invention is that in a given spherulizing furnace, for a given fuel feed rate and for a given bead composition, the production yield of beads can be 50 to 100% higher than when crushed glass cullet is used.
The most important factors influencing the bead structure are the volatile, gas evolving, content of the glass-former particles feeding the spherulizing furnace, and the granulometry of such particles. A high content of volatile substance will in itself encourage a tendency towards cellulation of the beads. The volatile content of a given glass-former composition can to some extent be adjusted by a simple drying step. However the granulometry plays a very important role. It has surprisingly been found that various glass-former compositions which convert to hollow beads when particles of such compositions are subjected to given spherulizing conditions will, despite the evolution of gas from within the particles, convert to solid beads under the same spherulizing conditions if the glass-former composition is fed to the spherulizing furnace in the form of smaller particles. How small the particles should be for that purpose, when using a given glass-former composition, depends on the temperature/time curve of the heat treatment in the spherulizing furnace. The appropriate maximum particle size can easily be determined by tests.